The present invention relates to ceramic articles based on solid solutions of beta-spodumene and a method of fabricating the same. Specifically, the invention relates to beta-spodumene ceramics having low thermal expansion, high porosity and high strength for use in high temperature applications, including automotive emissions control systems, and a method of fabricating such ceramics absent a glass component.
Cordierite ceramics have found broad use in automotive emissions control systems, including but not limited to automotive catalytic converter substrates and diesel particulate filters. When applied with a high-surface-area “washcoat” and noble metal catalysts, a cordierite catalytic converter device is highly efficient at reducing emission of CO, hydrocarbons, and nitrogen oxides (NOx) once the converter has been heated to a critical temperature by the exhaust. However, during the initial heat-up of the exhaust system, the converter is too cold to function adequately, and substantial amount of “cold-start” emissions are released from the tail pipe. Increasingly stringent government legislation for mobile emissions have resulted in efforts to reduce the time to light-off, and resulting “cold-start” emissions. One solution has been the emergence of substrates with thinner walls and increased porosity to lower the thermal mass (heat capacity per unit volume) of the converter for faster heat-up. However, new “ultra-thin-wall” cordierite substrates which have typical cell densities of 900 cells/inch2 (cpsi) or more, and wall thickness of 0.0027 inches (2.7 mils) or less, have insufficient strength. High strength is required to survive handling, canning and vibration in use.
For diesel particulate filter, cordierite has been the cost-effective material of choice due to its combination of low CTE, good thermal shock resistance, good strength, filtration efficiency, and low cost. However, the pressure of cordierite filters has been unsatisfactory high. A recent trend in the efforts to reduce the pressure drop in cordierite filters has been to increase the porosity. However, the increased porosity in combination with the inherent microcracking of the cordierite structure limit the strength thereof. A higher porosity would also be advantageous for the application of a catalyst system to the filter. Similarly to catalytic converter substrates, catalyst systems for filter use promote the conversion of carbon monoxide and unburned hydrocarbons in the exhaust stream to carbon dioxide and water. These catalyst systems are generally comprised of a mixture of highly dispersed noble metal catalyst supported on a high surface area metal oxide, and typically reside primarily within the pores of the filter walls to form a thin coating therein.
Lithium aluminosilicate (LAS) ceramics are known for combining low thermal expansion with high strength, and thermal durability. U.S. Pat. Nos. 3,600,204, 3,839,001, 5,403,787 and 5,962,351 disclose ceramics based on solid solutions of beta-spodumene for heat regenerator applications. LAS ceramics of this kind may therefore be suitable for application in automotive catalytic converter systems and diesel particulate filters. However, a disadvantage associated therewith is a high level of lithium in the structure. It has been found that lithium acts to poison the aforementioned noble metal catalyst systems. Another disadvantage resides in the fabrication of these structures which involves an extrusion process of batches based on glass powders. Glass batches are difficult to extrude requiring large amounts of binders and plasticizers. Also, it is difficult to control a uniform crystallization process.
It would be advantageous to employ beta-spodumene based ceramics for automotive catalytic converter substrate and diesel particulate filter applications, and to manufacture such articles without the use of a glass component. It would also be advantageous to obtain beta-spodumene based ceramic articles having lower levels of lithium for improved catalyst application and performance, while maintaining high strength, low thermal expansion and high porosity.